Hydrogen Bonding

[Shannon Moore 2L]

How do you find the number of sites on a molecule that are capable of hydrogen bonding?
I was looking at this question from the zoom review if it helps to understand the context.

The DNA double helix structure was determined in 1953 and found
to involve intermolecular H-bonding between DNA bases present within
the helix.
Below are two DNA-bases: Adenine (A) and Thymine (T)
For each base give the maximum number of possible H-bonding sites
with water molecules. (Lone pairs are not included in the structures.)

How do both of these molecules have 8 sites for hydrogen bonding?

[Thomas Vu 1A]

I would just remember that any molecules that contain a NOF with a lone electron pair can act as a H-bond site. Also, any H in a polar covalent bond is also available for H-bonding. Those are the main rules basically

[luludaly2B]

I had the same question. I keep counting 7 for both. Here is an image:

For A, there are 5 nitrogens each with one lone pair, and 2 carbons that can accept an H.
For G, there are 2 nitrogens, 2 oxygens each with 2 lone pairs, and one carbon. What am I missing?

[Mohamed Mido]

Any NOF atom that has lone pairs can H-bond. Alos all hydrogens bonded to NOF can H-Bond. It can be tricky because Hydrogen atoms that are bonded to Carbon can't H-bond, but NOF atoms bonded to Carbon can, so you'll have to remember that.

[Isabelle Hales 1J]

I had the same question. I keep counting 7 for both. Here is an image:
Screen Shot 2020-12-12 at 5.38.34 PM.png
For A, there are 5 nitrogens each with one lone pair, and 2 carbons that can accept an H.
For G, there are 2 nitrogens, 2 oxygens each with 2 lone pairs, and one carbon. What am I missing?

Hi! I think what you are missing is that carbon does not generally participate in hydrogen bonding because it does not have a particularly high electronegativity.
For A, you are correct with counting 5 for each lone pair on the nitrogens. I would also count the 3 hydrogen atoms that can bond to other molecules, which puts the total at 8. For B, you are correct in counting 2 for the nitrogen lone pairs, and 4 for the two oxygens. By counting the 2 hydrogen atoms as well, you will get to 8!

[Gicelle Rubin 1E]

Any NOF atom that has lone pairs can H-bond. Alos all hydrogens bonded to NOF can H-Bond. It can be tricky because Hydrogen atoms that are bonded to Carbon can't H-bond, but NOF atoms bonded to Carbon can, so you'll have to remember that.

Thank you for this!

[205323697]

If a molecule is non-polar, would it still having these hydrogen bonding site if this is even possible?